What is the difference between the symbols with decimal indices #45 and #127 from the T1 encoding? Character #45 seems to be the ordinary hyphen, but what about #127? The en-dash and em-dash are at positions #21 and #22, respectively.

It's evident that the normal hyphen is slightly asymmetric with respect to its bounding box and that the "protruding hyphen" has most of it outside its bounding box.

By setting \hyphenchar\font="7F (when the current font is T1 encoded) one gets these consequences

Explicit hyphens (the normal character "2D) won't insert discretionaries, nor will the ligatures starting with it

The hyphenation algorithm, when applied, would use the "protruding hyphen".

If the language establishes hyphenation rules where the normal hyphen can be used as break point, there wouldn't be two characters: the - followed by the "protruding hyphen" added by the hyphenation procedure would be merged into a single protruding hyphen.

The last point is realized by the second ligature

(LABEL O 55)
(LIG O 55 O 25)
(LIG O 177 O 177)
(STOP)

pertaining to character "octal 55" (that is, decimal 45 and hexadecimal "2D), which is shown in the last bit of the example. The log file reports

This character was included in the T1 encoding when microtypographical features were not even dreamt of (1990); as far as I know, it has never been really exploited. See M. Ferguson's article on TUGboat for more information.